Metal Mold, Optical Device, Metal Mold for Forming Optical Device, and Process for Manufacturing the Same

ABSTRACT

A mold for forming an optical element includes: a surface including a plurality of grooves arranged concentrically. Each of the grooves includes a bottom surface with a predetermined width, a side surface, and a curved surface with a predetermined curvature radius. The bottom surface extends in a radius direction of the grooves. The curved surface connects the bottom surface and the side surface.

TECHNICAL FIELD

The present invention relates to a mold, an optical element, a mold forforming the optical element and its manufacturing method. In particular,the present invention relates to a mold appropriate for processing amold for forming an optical element, an optical element formed by theaforesaid mold, a mold for forming an optical element, and amanufacturing method for the aforesaid mold.

BACKGROUND ART

In recent years, in a highly accurate optical element such as anobjective lens for an optical pickup apparatus, a diffractive structureis formed on its optical surface to improve an optical property by itsdiffractive effects. In this case, the diffractive structure isgenerally a microscopic structure of ring-shaped zones. When anobjective lens is formed through injection molding with plastic as amaterial, it is necessary to process the mold to form amicroscopic-groove form that corresponds to the structure of ring-shapedzones on its mold. As a material for the mold, cemented carbide isgenerally used, and this cemented carbide can be cut by a diamondcutting tool. Patent Literature 1 discloses a technology wherein amaterial of the mold is cut by a diamond cutting tool having arectangular cutting face while the material is rotated, to form themicroscopic-groove form.

Patent Literature 1: Unexamined Japanese Patent Application PublicationNo. 2005-219185

DISCLOSURE OF INVENTION

In this technology, when cutting force is applied on a cutting face ofthe diamond cutting tool, there is a problem that a tip corner portionof the tool tends to cleave in the direction of 45° and to be chippedoff due to crystal orientation of diamond. In the technology of PatentLiterature 1, when the corner portion of the cutting face is chippedoff, a surface inclining at 45° that follows a shape of the chippedportion is generated between a bottom surface of the groove and a sidesurface of the groove. When optical elements are molded through the moldhaving a groove shape of this kind, deformation is caused to follow thesurface including at 45° on the diffractive structure, which results ina fear of a decline of diffraction efficiency. On the other hand, thecleavage does not sometime appear depending on a diamond cutting tool.In that case, the surface inclining at 45° is not caused on the moldwhich is processed and formed by the tool. Further, deformation is notcaused on a diffractive structure of an optical element molded by theaforesaid mold, thus, diffraction efficiency can be kept to be high.Namely, in the prior art, accuracy of a mold varies by cleavage of adiamond tool that is an unexpected phenomenon, which causes a problemthat optical property (diffraction efficiency) of optical elementsvaries greatly depending on molds. On the other hand, it is possible toconsider that the target maximum value of diffraction efficiency ownedby an optical element is determined to be low in advance. However, it isdifficult to fundamentally solve the aforesaid variation caused bycleavage of the tool to be low.

The present invention is achieved in view of the aforesaid problems andis to provide a mold, an optical element, a mold for forming the Opticalelement and a method manufacturing the mold, whereby necessaryefficiency is secured, for example, in optical elements equipped withdiffractive structures and whereby variation in the formed productscaused in each mold for manufacturing the optical elements can becontrolled.

The above objects are achieved by embodiments described below.

1. A mold for forming an optical element including a plurality ofring-shaped zones, the mold comprising:

a surface comprising a plurality of ring-shaped zones arrangedconcentrically,

wherein each of the ring-shaped zones comprises a groove comprising abottom surface having a predetermined width extending in a radiusdirection of the ring-shaped zones, a side surface, and a curved surfacewith a curvature radius in a range from 0.5 μm to 2 μm connecting thebottom surface and the side surface.

According to the mold relating to the present invention, ring-shapedzones with the following structures are formed on a mold in at least afirst cutting process with, for example, a diamond cutting tool. Each ofthe ring-shaped zones includes a groove bottom surface and a groove sidesurface which are connected through a curved surface with a curvatureradius in a range from 0.5 μm to 2 μm. It secures a required opticalproperty of an optical element formed through a transfer process withthe mold. Further, it reduces a change of a curvature radius of thecurved surface between the start and the end of the cutting process, andcontrols a variation of optical elements molded by the molds processedas above depending on the molds. Herein, “curvature radius” represents avalue measured on a cross section along the optical axis. Further,“comprising a bottom surface (or top portion), a side surface, and acurved surface with a curvature radius in a range from 0.5 μm to 2 μmconnecting the bottom surface and the side surface” described above notonly means that all of the plurality of ring-shaped zones which areformed as above are constructed by curved surfaces, but also means thatcurved surfaces connect the surfaces in a part of the ring-shaped zones,because of an usable efficiency. For example, said curved surfaces mayconnect the surfaces in 50% or more of the whole ring-shaped zones.

2. The mold of Item 1, wherein the bottom surface is formed in a shapewith a plurality of steps.

The above mold is preferable to form a groove structure in whichring-shaped zones in stair shape are repeatedly arranged around opticalaxis X as shown in FIG. 1( a).

3. The mold of Item 1, wherein the bottom surface does not comprise astep.

The above mold is preferable to form a groove structure in which aplural side walls with the same height forming ring-shaped zones arearranged around optical axis X as shown in FIG. 1( b).

4. An optical element comprising:

an optical surface comprising a plurality of ring-shaped zones formedconcentrically around an optical axis, each of the ring-shaped zonescomprising a top portion having a predetermined width extending in adirection perpendicular to the optical axis, a side surface, and acurved surface with a curvature radius in a range from 0.5 μm to 2 μmconnecting the top portion and the side surface.

An optical element according to the present invention can be formed witha mold described in Item 1, thus, exhibits the same action and effect tothe mold.

5. The optical element of Item 4, wherein each of the ring-shaped zonescomprises a stepped structure formed in a stair shape.

As for the optical element described in Item 5, each of the plurality ofring-shaped zones in the embodiment described in Item 4 includes astepped structure formed in a stair shape. A stepped structure formed ina stair shape means a structure in which ring-shaped zones in stairshape are repeatedly arranged around optical axis X as shown in FIG. 1(a).

6. The optical element of Item 4,

wherein each of the ring-shaped zones is formed in a binary structure.

As for the optical element described in Item 6, each of the plurality ofring-shaped zones in the embodiment described in Item 4 is formed in abinary structure. A binary structure means a structure in which a pluralside walls with the same height forming ring-shaped zones are arrangedaround optical axis X as shown in FIG. 1( b).

7. A method for manufacturing a mold for forming an optical element, themold comprising a mold base-material on which a plurality of grooves areformed concentrically by relatively driving a diamond cutting tool andthe mold base-material for cutting the mold base-material, the diamondcutting tool comprising a cutting edge with a cutting face and a flank,the mold base-material being driven to be rotated, each of the groovescomprising a bottom surface having a predetermined width extending in aradius direction of the grooves, the method comprising:

a first step of cutting the mold base-material while relatively drivingthe diamond cutting tool or the mold base-material in a direction of arotation axis of the mold;

a second step of cutting the mold base-material while relatively drivingthe diamond cutting tool or the mold base-material in a directionperpendicular to the rotation axis of the mold; and

a third step of cutting the mold base-material by displacing the diamondcutting tool or the mold base-material simultaneously in a direction ofthe rotation axis of the mold and a direction perpendicular to therotation axis of the mold while relatively driving the diamond cuttingtool or the mold base-material, between the first and the second steps.

The manufacturing method relating to the present invention, includes thethird step of cutting the mold base-material by displacing the diamondcutting tool or the mold base-material simultaneously in a direction ofthe rotation axis of the mold and a direction perpendicular to therotation axis of the mold while relatively driving the diamond cuttingtool or the mold base-material, between the first and the second steps.Therefore, a predetermined surface crossing the bottom surface and theside surface of a groove can be formed at the intersection of the bottomsurface and the side surface. It reduces a change of a curvature radiusof the curved surface between the start and the end of the cuttingprocess, and controls a variation of optical elements molded by themolds processed as above depending on the molds.

8. The method of Item 7 for manufacturing a mold for forming an opticalelement, wherein the mold base-material is cut in the third step suchthat the bottom surface and a side surface are connected by apredetermined curved surface in the groove.

According to the embodiment described in Item 8, a change of a curvatureradius of the curved surface between the start and the end of thecutting process can be reduced, and a variation of optical elementsmolded by the molds processed as above depending on the molds can becontrolled.

9. The method of claim 8 for manufacturing a mold for forming an opticalelement, wherein a curvature radius of the curved surface is in a rangefrom 0.5 μm to 2 μm.

The method described in Item 9 for manufacturing a mold for forming anoptical element, according to the embodiment of Item 7, includes thecurved surface with a curvature radius a range from 0.5 μm to 2 μm. Itsecures a required optical property of an optical element formed throughtransferring step with the manufactured mold for molding an opticalelement.

10. A method for manufacturing a mold for forming an optical element byrelatively driving a diamond cutting tool and a mold base-material forcutting the mold base-material, the mold base-material being driven tobe rotated, the diamond cutting tool comprising a cutting edgecomprising a cutting face comprising a first edge portion, a second edgeportion, and a third edge portion crossing the first edge portion andthe second edge portion, each of the first edge portion and the secondedge portion being in a line shape and extending parallel to each other,the mold comprising a mold base-material on which a plurality of groovesare arranged concentrically, each of the grooves comprising a bottomsurface having a predetermined width extending in a radius direction ofthe grooves, the method comprising:

a first step of cutting the bottom surface of each of the grooves on themold base-material by the third edge portion of the diamond cutting toolby relatively driving the diamond cutting tool or the mold base-materialin a direction of the rotation axis of the mold;

a second step of cutting the side surface on the mold base-material bythe second edge portion of the diamond cutting tool by relativelydriving the diamond cutting tool or the mold base-material in adirection perpendicular to the rotation axis of the mold; and

a third step of cutting the mold base-material by displacing the diamondcutting tool or the mold base-material simultaneously in the directionof the rotation axis of the mold and the direction perpendicular to therotation axis of the mold, between the first and second steps.

In the manufacturing method relating to the invention, includes thethird step of cutting the mold base-material by displacing the diamondcutting tool or the mold base-material simultaneously in a direction ofthe rotation axis of the mold and a direction perpendicular to therotation axis of the mold while relatively driving the diamond cuttingtool or the mold base-material, between the first and the second steps.Therefore, a curved surface crossing the bottom surface and the sidesurface of a groove can be formed at the intersection of the bottomsurface and the side surface. It reduces a change of a curvature radiusof the curved surface between the start and the end of the cuttingprocess, and controls a variation of optical elements molded by themolds processed as above depending on the molds.

As for “optical elements” herein, there are given, for example, a lens,a prism, a diffractive grating optical element (diffractive lens,diffractive prism, diffractive plate and chromatic aberration correctingelement), an optical filter (spatial lowpass filter, wavelength bandpassfilter, wavelength lowpass filter, wavelength highpass filter andothers), a polarization filter (analyzer, azimuth rotator, polarizationseparating prism or the like) and a phase filter (phase plate, hologramor the like), to which, however, the invention is not limited.

In the invention, it is possible to provide a mold, an optical element,a mold for molding an optical element and its manufacturing method whichare epochal not to be obtained by the prior art and which can controlvariation of, for example, optical elements with diffractive structurescaused per respective molds for molding the optical elements, whilesecuring required efficiency in the optical elements.

BRIEF DESCRIPTION OF DRAWINGS

Each of FIG. 1( a) and FIG. 1( b) shows an example of a diffractivestructure.

FIG. 2 is a perspective view of biaxial superfine processing machine 10that is suitable for carrying out a method of processing a mold relatingto the present embodiment.

FIG. 3 is a perspective view of a diamond cutting tool.

FIG. 4( a) is a diagram of diamond cutting tool T that is suitable to beused for a processing method relating to the invention that is viewedfrom a cutting face on the tip, and FIG. 4( b) is a side view of theforegoing.

FIG. 5 is a perspective view of mold MD which has been processed by thediamond cutting tool shown in FIG. 4( a) and FIG. 4( b).

FIG. 6 is a schematic enlarged sectional view of an optical transfersurface of mold material M.

Each of enlarged FIGS. 7( a)-7(c) shows a position of diamond cuttingtool T in the case of processing.

FIG. 8( a) shows an enlarged area around a groove bottom surface of amold cut by a diamond cutting tool through a processing method of thepresent embodiment, and FIG. 8( b) shows an enlarged area around agroove side surface of a mold cut by a diamond cutting tool through aprocessing method of the present embodiment.

FIG. 9( a) shows an enlarged area around a groove bottom surface of amold cut by a diamond cutting tool through a processing method in thecomparative example, and FIG. 9( b) shows an enlarged area around agroove side surface of a mold cut by a diamond cutting tool through aprocessing method in the comparative example.

FIG. 10 is a sectional view for an example of optical element OE thatcan be used for an optical pickup apparatus.

FIG. 11 is a diagram showing results of simulations conducted byinventors of the present invention.

FIG. 12 is a diagram showing results of simulations conducted byinventors of the present invention.

FIG. 13 is a diagram showing results of simulations conducted byinventors of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the invention will be explained as follows, referringto the drawings. FIG. 2 is a perspective view of biaxial superfineprocessing machine 10 that is suitable for carrying out a method ofprocessing a mold relating to the present embodiment, and FIG. 3 is aperspective view of a diamond tool. In the biaxial superfine processingmachine 10 shown in FIG. 2, X-axis table 2 that is driven in the X-axisdirection by an unillustrated control device is arranged on pedestal 1.On the X-axis table 2, there is mounted diamond cutting tool T. Further,Z-axis table 4 that is driven in the Z-axis direction by anunillustrated control device is arranged on pedestal 1. On Z-axis table4, there is mounted main shaft (rotation axis) 5 that is driven to berotated by an unillustrated control device. On the main shaft 5, a moldfor forming optical elements each having a transfer optical surface tobe processed, can be mounted. The diamond cutting tool T is equipped onits tip with diamond chip Tc whose shape will be explained later withreferring to FIGS. 4( a) and 4(b).

A processing method relating to the present embodiment makes it possibleto create grooves in a form of ring-shaped-zones corresponding to thediffractive structure by conducting cutting process in the ductilitymode under the conditions wherein a superfine processing machine whosemain shaft 5 and X/Z axes tables 2 and 4 are stiff extremely and shaftcontrol resolving power is 100 nm or less is used, then, a mold formolding optical elements representing a work is clamped on the mainshaft 5, and the main shaft 5 is rotated at a rotating speed 1000 min⁻¹so that a cutting point of a cutting blade may be moved continuously inthe course of processing by diamond cutting tool T on conditions of anamount of cutting of 1 μm and a feed mount of 0.2 mm/min.

FIG. 4( a) is a diagram of diamond cutting tool T that is viewed fromcutting face T5 at the tip thereof and is favorably used in a processingmethod for a mold relating to the present embodiment, and FIG. 4( b) isa side view of the diamond cutting tool T. In FIG. 4( a), the diamondcutting tool T has therein third edge portion T3 (100 μm or less) on thetip that linearly extends, and first edge portion T1 and second edgeportion T2 which linearly extend in parallel from opposing ends of thethird edge portion T3 in the direction perpendicular to the third edgeportion T3. These edge portions form cutting face T5 in a rectangularform. The first edge portion T1 is connected to fourth edge portion T4that extends to be away from the second edge portion T2. It ispreferable that a straight edge portion of the second edge portion T2and that of the third edge portion T3 intersect at right angles eachother ideally, but both edge portions may be connected by a microscopiccircular arc R1 with radius A (less than 0.5 μm). The rectangular formin this case is defined as an area surrounded by edge portions T1, T2,T3 and line T6, where the second edge portion T2 is shorter in the edgeportions T2 and T3 which extend to be in parallel each other, and theline T6 is drawn to be parallel to the first edge portion T1 from an endportion (position that is away from a tip by distance P) of the secondedge portion T2 at the fourth edge portion T4 side.

FIG. 5 is a perspective view of mold MD which has been processed by thediamond cutting tool shown in FIG. 4( a) and FIG. 4( b), and FIG. 6 is aschematic enlarged sectional view of an optical transfer surface ofmaterial M of the mold.

Each of enlarged FIGS. 7( a)-7(c) shows a position of diamond cuttingtool T in the case of processing. It is preferable that mold MDprocessed by a processing method of the present embodiment is used forforming optical elements each having a diffractive structurerepresenting a structure wherein an optical functional surface on atleast one side is divided into plural optical functional areas whosecenters are on an optical axis, and at least one of the plural opticalfunctional areas is divided into plural areas in a ring-shaped zoneswhose centers are on the optical axis, then, discontinuous steps inprescribed number are provided on each ring-shaped zone, and ring-shapedzones where discontinuous steps are provided are arranged continuously.

FIG. 10 is a sectional view for an example of optical element OE thatcan be used for an optical pickup apparatus. A diffractive structure isdrawn in exaggeration in the sectional view. In FIG. 10, diffractivestructure DS1 and diffractive structure DS2 are formed respectively onoptical surface S1 and optical surface S2 of optical element OE andexhibit diffractive effects for a light flux passing through thediffractive structures. The diffractive structures DS1 and DS2 areformed concentrically around optical axis O, and they are composed ofring-shaped zones each having a top portion with a predetermined widthextending in the direction perpendicular to the optical axis. They have,for example, a stair shape on a cross section taken in the optical axisdirection. Grooves of ring-shaped zones are formed in stair shape withplural steps on an optical transfer surface of mold MD as shown in FIG.6, so as to complement the aforesaid groove structures, where theoptical transfer surface means a surface for transferring an opticalsurface of an optical element and this surface is preferably plated withnickel, phosphorus or copper. Incidentally, in FIG. 6, diamond cuttingtool T is movable in X-axis direction (which is also referred as adirection toward the rotation axis) and in Z-axis direction (which isalso referred as a direction parallel to the rotation axis of the mold).

An example of a processing method of the present embodiment will bedescribed below. Processing in the present embodiment is effective whenforming a groove structure in a shape of ring-shaped zone (which is alsoreferred as grooves or grooves of ring-shaped zones simply) equippedwith plural steps which become gradually deeper. Here, forms of groovestructures to be formed on a mold are inputted in a control device of asuperfine processing machine in advance. In the example shown in FIG. 6,groove structure G1 and groove structure G2 are formed on an opticaltransfer surface in order from optical axis O to the outercircumference. Steps in stair shape (which is also called as astaircase) of each of groove structure G1 and groove structure G2becomes deeper toward the outer circumference. Herein, “shallow” or“deep” is relatively distinguished depending on a depth of cut in adirection of the rotation axis when the diamond cutting tool cuts thesurface of a mold to be processed. Further, “cutting direction” is inparallel to the rotation axis and a direction to approach the mold.

When processing a mold, mold material M is rotated around optical axis O(which is also called a rotation axis line), and third edge portion T3of diamond cutting tool T is moved to the prescribed position which is astandard position (the position shown with a solid line in FIG. 6)corresponding to the fourth step in groove structure G2 (the deepeststep in groove structure G2) in this case, in the direction parallel tooptical axis O and the direction to further press the tool against moldmaterial M (downward direction along Z-axis). Owing to this, diamondcutting tool T cuts an optical transfer surface of mold material M toform grooves of a ring-shaped zone. Groove bottom surface (that is alsoreferred as a bottom surface of the groove) GB is cut by third edgeportion T3, while, groove side surface (that is also referred as abottom wall surface of the groove) G0 of outer circumstance side is cutby first edge portion T1

(First Step: see FIG. 7( a)).

Next, as shown in FIG. 7( b), diamond cutting tool T is displaced in thedirection intersecting optical axis O of mold material M (rightdirection along X-axis) and in rotation axis line direction (upwarddirection along Z-axis) simultaneously (Third step). In this operation,curved surface section CP is formed between groove bottom surface GB andgroove side surface GI on the inner circumferential side due to circulararc section R1 between second edge portion T2 and third edge portion T3.Curvature radius of B of curved surface portion CP is in a range from0.5 μm to 2 μm.

Further, diamond cutting tool T is displaced in parallel with opticalaxis O of mold material M (upward direction along Z-axis) to cut grooveside surface G1 at the inner circumferential side by the second edgeportion T2 (second step). After that, diamond cutting tool T isdisplaced in parallel to optical axis O of mold material M (rightdirection along X-axis), as shown in FIG. 7( c), to form thesecond-deepest step. The diamond cutting tool T is further displaced toform groove structures G2 and G1 in the same way. As is apparent fromthe foregoing, when the diamond cutting tool T is drawn out of moldmaterial M and displaced in the X-axis direction each time a groove isformed repeatedly, it is possible to form mold MD that is suitable forforming a diffractive structure of a binary type (see FIG. 1( a)) Amethod of molding an optical element by using the mold MD of this kindis described, for example, in Unexamined Japanese Patent ApplicationPublication No. 2005-319778.

For example, it is possible to obtain optical element OE1 shown in FIG.10, by injecting melted material for an optical element into molds shownin FIG. 6 and FIGS. 7( a)-7(c) and by cooling it. When using a mold inwhich a plurality of ring-shaped zones having a stepped structure in astair-shape or a binary structure are formed on an optical transfersurface, a plurality of ring-shaped zones RZ having a step structure ina stair-shape or a binary structure are transferred onto an opticalsurface of an optical element by this mold. As an example of anembodiment of the invention, there is shown a diagram that a crosssection of a ring-shaped zone is enlarged, in a circle shown in FIG. 10.Each ring-shaped zone has top portion OET corresponding to groove bottomsurface GB of a mold and side wall surface OEO corresponding to grooveside surface G1 of a mold. The top portion OET and the side wall surfaceOE1 are connected by curved surface OECP that has a curvature radius ina range of 0.5-2 μm and that corresponds to curved surface portion CP ofthe mold. Further, the top portion has a predetermined width in thedirection perpendicular to the optical axis. In the circle in FIG. 10,there is shown a sectional view of a step structure in stair-shape.However, it is not limited to the shape in FIG. 10, as far as a shape ofthe ring-shaped zone of the optical element relating to the inventionincludes a top portion having a predetermined width in the directionperpendicular to the optical axis, a side wall surface, and a curvedsurface having a curvature radius in a range of 0.5-2 μm which connectsthe top portion and the side wall surface.

Further, as a material of the optical element relating to the invention,it is possible to utilize a general glass optical material, withoutbeing limited to plastic.

An optical element relating to the invention may also be molded throughpress molding, injection molding, mold casting and other known moldingmethod that molds an optical element, by using a mold relating to theinvention.

In the meantime, in the course of cutting process, there is a fear thata corner of cutting face T5 of diamond cutting tool T is lost throughcleavage by cutting resistance. In this case, a shape of lost cuttingface T5 is transferred to mold material M. An optical element formedthrough transferring by the mold of this kind is feared that desiredoptical properties cannot be exhibited because the optical element has adiffractive structure that is different from a design shape. Theproblem, in particular, is variation in shapes of optical elementsformed by plural molds, depending on the molds. Namely, it causes a fearthat diffraction efficiency of an optical element molded by a certainmold is greatly different from diffraction efficiency of an opticalelement molded by another mold, resulting in an increase of percentdefective.

In contrast to this, in the present embodiment, it is possible tolighten these troubles, because a groove bottom surface of a mold isconnected to a groove side surface of a mold through a curved surface.This will be explained as follows, referring to the drawings, under thecondition that a groove structure of a mold can be completelytransferred with respect to a diffractive structure of an opticalelement.

FIG. 8( a) shows an enlarged area around a groove bottom surface of amold cut by a diamond cutting tool through a processing method of thepresent embodiment, while, FIG. 8( b) shows an enlarged area aroundgroove side surface of a mold cut by a diamond cutting tool through aprocessing method of the present embodiment. FIG. 8( a) shows a grooveat the start of cutting (the start of processing) and FIG. 8( b) shows agroove at the end of cutting (the end of processing). FIG. 9( a) showsan enlarged area around a groove bottom surface of a mold cut by adiamond cutting tool through a processing method of the comparativeexample, while, FIG. 9( b) shows an enlarged area around a groove sidesurface of a mold cut by a diamond cutting tool through a processingmethod of the comparative example. FIG. 9( a) shows a groove at thestart of cutting (the start of processing) and FIG. 9( b) shows a grooveat the end of cutting (the end of processing).

In FIGS. 8( a) and 8(b), in this case, a light flux which has passedthrough the optical surface on which the groove bottom surface CB istransferred, emerges at a desired diffraction angle, but a light fluxwhich has passed through the optical surface on which the curved surfaceCP is transferred does not emerge at a desired diffraction angle, tobecome unwanted light. Therefore, it is preferable to make an area ofcurved surface CP to be zero originally, for enhancing diffractionefficiency of the optical element. The ideal groove structure of thatkind (a form shown in FIG. 9( a)) can be acquired by cutting withdiamond cutting tool T wherein second edge portion T2 and third edgeportion T3 intersect each other through straight lines.

However, if chipped portion D appears in diamond cutting tool T duringthe period from the start to the end of cutting due to cleavageinclining to the third edge portion T3 in the direction of 45°, taperedsurface TP wherein chipped portion D is transferred on the cornersection appears in the groove structure at the end of cutting, as shownwith a solid line in FIG. 9( b). In this case, a light flux passingthrough an optical surface on which the tapered surface TP istransferred, does not emerge at a desired diffraction angle and becomesunwanted light, thus, diffraction efficiency is lowered by an amountequivalent to the area of the tapered surface TP. This is a cause ofoccurrence of variation of diffraction efficiency before and after theoccurrence of chipped portion D. Incidentally, there are circumstancesthat it cannot be judged whether the chipped portion D appears or not inthe course of processing.

In contrast to this, in the present embodiment, curved surface CP shownin FIG. 8( a) is generated at the start of cutting, whereby, diffractionefficiency of optical elements formed through transferring the curvedsurface CP is lower than diffraction efficiency of optical elementsformed through transferring by the mold of a groove structure shown inFIG. 9( a). However, a difference of an area viewed in the direction ofa rotation axis line between curved surface CP before occurrence of thechipped portion D and curved surface CP′ after occurrence of the chippedportion D is small. Therefore, variation of diffraction efficiency canbe controlled in optical elements formed through transferring by themolds of that kind.

Incidentally, if curvature radius B of curved surface CP is increased,it reduces the difference of the area viewed in the direction of arotation axis line between curved surface CP before occurrence of thechipped portion D and curved surface CP′ after occurrence of the chippedportion D on a diamond cutting tool. Therefore, variation of lightutilization efficiency (diffraction efficiency) for optical elementsformed through transferring by the molds of that kind becomes smaller.In contrast to this, an absolute value of the light utilizationefficiency for optical elements formed through transferring by the moldof that kind is lowered, because a difference of areas between curvedsurface CP and curved surface CP′ in a shape (FIG. 9( a)) of the moldwherein a tool is not worn out and diffraction efficiency is 100%,becomes greater. It is therefore necessary to establish curved surfaceCP that restrains variation of light utilization efficiency, whilesecuring the absolute value of light utilization efficiency that isnecessary for optical elements.

When considering that a value of actual results of variation in anamount of abrasion of a diamond cutting tool is 0-0.5 μm, at least 0.5μm of curvature radius of B of curved surface CP can reduce thevariation of light utilization efficiency effectively. Further, if thecurvature radius B is 2 μm or less, an absolute value of lightutilization efficiency necessary for an optical element can be secured.Therefore, optical properties necessary for optical elements formedthrough transferring by the mold relating to the invention can besecured by making curvature radius B to be in the range of 0.5 μm 2 μm.

Each of FIGS. 11, 12 and 13 is a diagram showing results of simulationscarried out by the present inventors, and showing relationship betweenan amount of abrasion of the tool (a length of chipped portion of thirdedge portion T3) and diffraction efficiency of optical elements formedthrough transferring from a mold formed by the tool. FIG. 11 is thediagram of a mold in the conventional form, FIG. 12 is the diagram of amold that is in a conventional form but is designed to lower the maximumdiffraction efficiency of an optical element to 95%, and FIG. 13 is thediagram showing results of simulations wherein optical elements formedand transferred from the mold in the present embodiment are used. In thepresent simulation, curvature radius B was made to be 0.5 μm. Further,in FIGS. 11-12, the vertical axis represents diffraction efficiency (%)of optical elements, and the horizontal axis represents an amount ofabrasion (μm) of the tool. As for optical elements formed throughtransferring from the mold of the comparative example shown in FIG. 9(a) and FIG. 9( b), diffraction efficiency under the state where noabrasion is caused (design efficiency) is 100% as shown in FIG. 11, butthe diffraction efficiency varies as greatly as 11% if tool abrasion of0.5 μm is caused. For this problem, it is also possible to consider themold design in which the maximum diffraction efficiency of the opticalelement is lowered to restrain the variation of diffraction efficiency.However, the simulation conducted by inventors of the present inventionclarified that the variation of diffraction efficiency in the case oftool abrasion of 0.5 μm remains to be 11% even when the mold is designedso that diffraction efficiency under the condition of no abrasion oftool (design efficiency) may be 95%, as shown in FIG. 12. The simulationfurther clarified that minimum diffraction efficiency is lowered andproperties of optical elements are worsened. On the other hand, as foroptical elements formed through transferring from the mold of thepresent embodiment shown in FIGS. 8( a) and 8(b), diffraction efficiencyunder the condition of no abrasion of tool (design efficiency) is 95%,and the variation of diffraction efficiency can be restrained to 7% evenwhen tool abrasion of 0.5 μm is caused. It enhances overall diffractionefficiency and secures necessary optical properties, although themaximum diffraction efficiency is lowered slightly.

Though an explanation of the invention has been given by reference tothe embodiment of the invention, the invention should not be construedto be limited to the aforesaid embodiment, and it is naturally possibleto modify and improve properly. For example, the processing methodrelating to the invention can also be used for those other thanprocessing of a mold for forming optical elements. Further, a shape of acutting face of a diamond cutting tool used for the processing method ofthe invention may also be a tapered form without being a rectangularshape. Further, the mold may be processed by displacing a diamondcutting tool from an inner side to an outer side along the directionperpendicular to the optical axis and the rotation axis line.

1. A mold for forming an optical element including a plurality ofring-shaped zones, the mold comprising: a surface comprising a pluralityof ring-shaped zones arranged concentrically, wherein each of thering-shaped zones comprises a groove comprising a bottom surface havinga predetermined width extending in a radius direction of the ring-shapedzones, a side surfaces and a curved surface with a curvature radius in arange from 0.5 μm to 2 μm connecting the bottom surface and the sidesurface.
 2. The mold of claim 1, wherein the bottom surface is formed ina shape with a plurality of steps.
 3. The mold of claim 1, wherein thebottom surface does not comprise a step.
 4. An optical elementcomprising: an optical surface comprising a plurality of ring-shapedzones formed concentrically around an optical axis, each of thering-shaped zones comprising a top portion having a predetermined widthextending in a direction perpendicular to the optical axis, a sidesurface, and a curved surface with a curvature radius in a range from0.5 μm to 2 μm connecting the top portion and the side surface
 5. Theoptical element of claim 4, wherein each of the ring-shaped zonescomprises a stepped structure formed in a stair shape.
 6. The opticalelement of claim 4, wherein each of the ring-shaped zones is formed in abinary structure.
 7. A method for manufacturing a mold for forming anoptical element, the mold comprising a mold base-material on which aplurality of grooves are formed concentrically by relatively driving adiamond cutting tool and the mold base-material for cutting the moldbase-material, the diamond cutting tool comprising a cutting edge with acutting face and a flank, the mold base-material being driven to berotated, each of the grooves comprising a bottom surface having apredetermined width extending in a radius direction of the grooves, themethod comprising: a first step of cutting the mold base-material whilerelatively driving the diamond cutting tool or the mold base-material ina direction of a rotation axis of the mold; a second step of cutting themold base-material while relatively driving the diamond cutting tool orthe mold base-material in a direction perpendicular to the rotation axisof the mold; and a third step of cutting the mold base-material bydisplacing the diamond cutting tool or the mold base-materialsimultaneously in a direction of the rotation axis of the mold and adirection perpendicular to the rotation axis of the mold whilerelatively driving the diamond cutting tool or the mold base-material,between the first and the second steps.
 8. The method of claim 7 formanufacturing a mold for forming an optical element, wherein the moldbase-material is cut in the third step such that the bottom surface anda side surface are connected by a predetermined curved surface in thegroove.
 9. The method of claim a for manufacturing a mold for forming anoptical element, wherein a curvature radius of the curved surface is ina range from 0.5 μm to 2 μm.
 10. A method for manufacturing a mold forforming an optical element by relatively driving a diamond cutting tooland a mold base-material for cutting the mold base-material, the moldbase-material being driven to be rotated, the diamond cutting toolcomprising a cutting edge comprising a cutting face comprising a firstedge portion, a second edge portion, and a third edge portion crossingthe first edge portion and the second edge portion, each of the firstedge portion and the second edge portion being in a line shape andextending parallel to each other, the mold comprising a moldbase-material on which a plurality of grooves are arrangedconcentrically, each of the grooves comprising a bottom surface having apredetermined width extending in a radius direction of the grooves, themethod comprising: a first step of cutting the bottom surface of each ofthe grooves on the mold base-material by the third edge portion of thediamond cutting tool by relatively driving the diamond cutting tool orthe mold base-material in a direction of the rotation axis of the mold;a second step of cutting the side surface on the mold base-material bythe second edge portion of the diamond cutting tool by relativelydriving the diamond cutting tool or the mold base-material in adirection perpendicular to the rotation axis of the mold; and a thirdstep of cutting the mold base-material by displacing the diamond cuttingtool or the mold base-material simultaneously in the direction of therotation axis of the mold and the direction perpendicular to therotation axis of the mold, between the first and second steps.